The 200N of tension is the literal stretch tension in the belt. That's the tension that the spring in the tensioner should apply before you lock it up, and should therefore also be the same tension in the belt after it is locked up, until it stretches a bit.
The 1/4 turn thing is going to vary according to the belt. Some might be a little more flexible than others. Just pushing straight on the belt should produce a little deflection, but not a lot, and you should be able to push pretty damn hard without the belt moving a lot. Beyond that, it's not really possible to do it up too tight unless you do the wrong thing with the hex key while tightening up the tensioner. You shouldn't be trying to add much force to what the spring provides.